1. Field of the Invention
The present invention relates to a gas concentration detecting apparatus for detecting a specific component concentration based on detection results from a gas concentration sensor and, more particularly, to an improved technique for suitably detecting abnormality in a sensor control system.
2. Description of the Related Art
One known gas concentration sensor is, for example, a limiting-current type air-fuel ratio sensor (referred to as A/F sensor), which takes an exhaust gas released from a vehicle engine as a detected gas and detects the oxygen concentration (air-fuel ratio) in the exhaust gas. Specifically, the A/F sensor has a sensor element that includes a solid electrolyte member and a pair of electrodes disposed on that solid electrolyte member. The A/F sensor is configured to flow an element current corresponding to the present concentration, which is caused by an applied voltage on the sensor element. The A/F sensor then measures the element current flow through the sensor element to detect the oxygen concentration (air-fuel ratio).
The above-described A/F sensor cannot accurately measure the element current or cannot detect the oxygen concentration (air-fuel ratio), if any abnormality occurs at the sensor element terminals, such as a short to the power supply (power supply short circuit), a short to the ground (GND short circuit), and a short across the terminals (terminal-to-terminal short circuit). The conventional A/F sensors, therefore, use the voltages at both terminals of the sensor element as an A/D input for a microcomputer, and detect the abnormality if the A/D input is out of a predetermined range. Receiving voltages at both terminals of the sensor to obtain an A/D input needs, however, extra A/D converters by the number of the sensor terminals. This configuration is very complicated. An improved technique is thus required.
The conventional technique shown in a Japanese application patent laid-open publication No. HEI 08 (1996)-271475, for example, monitors the A/F sensor activation state to detect a sensor abnormality. Specifically, the A/F sensor abnormality is detected by determining whether the A/F sensor element resistance detected is in a predetermined range, or by determining whether the power supply to the heater for element activation is in a predetermined range. That is, the sensor abnormality is detected by determining whether the sensor element is normally activated.
The above-described abnormality detection method in the Japanese application patent laid-open publication, however, determines the state where the sensor element should obviously be activated and detects the sensor element abnormality if the sensor element is not activated under the state. The method can thus precisely detect the abnormality only under the state where the sensor element is actually activated or the state where the sensor element can be definitely determined to be activated. This then places much restriction on the sensor element abnormality detection. The method for sensor element abnormality detection based on the power supply to the heater needs circuitry for measuring the heater power or A/D converter or the like. This makes it hard to simplify the configuration. These problems need more preferable technique for detecting abnormality in the sensor control system.